Handle errors thrown from functions

[Ajedi32]

Currently if a function throws an error each doesn't pass it on to metalsmith, but instead lets it bubble up to the top-level scope. Metalsmith each should instead catch the errors and forward them to Metalsmith via the done() callback.

[wilsaj]

I like the idea.

Thoughts on whether it should it just exit early, calling done() with the first error encountered, or instead collect all the errors and forward an array of all of them at the end?